Rack bars normally comprise a round bar with teeth cut or formed at a "toothed end", the remaining non-toothed region referred to as a "shank end". Variable ratio rack bars such as described in U.S. Pat. No. 3,753,378 incorporate teeth of varying cross-section and varying skew angle with respect to the rack bar longitudinal axis, as well as varying tooth pitch. The pinion which engages the rack is usually helical and is installed in the steering gear at an oblique angle to the normal to the rack bar longitudinal axis, henceforth termed the "pinion installation angle". For variable ratio rack bars the teeth at the toothed end of the rack bar are usually symmetrically disposed about a mid point, this "on-centre region" corresponding to the pinion meshing position when the vehicle is steered straight ahead.
The geometry of such teeth makes variable ratio rack bars very difficult to machine by known rack manufacturing processes, and precision forging is therefore often resorted to, notwithstanding the high precision that is required for satisfactory pinion meshing.
The optimum geometry of the teeth of such rack bars varies widely according to specific vehicle requirements, and whether the steering gear is of the power or manual type. When viewed in cross-section normal to the longitudinal axis of the rack bar, the teeth are frequently positioned towards the periphery of the bar so that the bending strength at the toothed end of the rack bar is minimally reduced as compared to that at the shank end.
U.S. Pat. No. 4,116,085 describes a rack bar having at its toothed end a cross section of triangular or "Y" section which is particularly suited for variable ratio racks. Such a rack may be formed in forging dies such as those described in U.S. Pat. No. 4,571,982 and U.S. Pat. No. 5,862,701, and is supported in the steering gear in a V shaped rack pad. The above-referenced United States patents are being incorporated by reference. This arrangement provide resistance to rack roll under the action of tooth meshing forces as described in U.S. Pat. No. 4,116,085. An additional advantage of the above Y section, and of the related manufacturing process, is that the cross-sectional area of the Y section (including the mean height of the teeth in the toothed region) may be made to match that of the round rack bar blank from which the rack is forged, thereby saving material and enabling the construction of a die in which there is no flash. This die construction enables very high forming pressures to be achieved, so that precise filling of the tooth cavities of the die may be achieved even if forging is conducted at relatively low temperatures, the latter which is also conducive to the avoidance of scaling. The Y section of the rack is of larger diameter over corners so that its strength in bending at the toothed end approximately matches that at the shank end. A suitable temperature has been found to be around 700.degree. C. (often referred to as "warm forging") in contrast to a temperature of 1100.degree. C. typically used in conventional hot forging.
This use of a Y section design has been found to be highly desirable in variable ratio power steering gears where a large change of steering ratio is appropriate to the vehicle handling characteristics and, as a result, the skew angle of some of the teeth is large. The use of a Y section serves to stabilise the rack bar under the influence of rolling moments caused by the pinion/rack tooth contact forces, in particular the lateral component of these forces caused by the presence of large skew angles. However, in certain circumstances, such as when the skew angles are relatively small, for example during the introductory phase of variable ratio in a particular vehicle model where lesser degrees of ratio change may be employed, it may be desirable to provide the more conventional round cross section at the toothed end of the rack bar so that the rack assumes a section like the letter "D", and the curved back of this section is of a radius substantially concentric with the radius of the shank end of the bar. This arrangement has some manufacturing advantages for power steering gears over racks of Y section in that the toothed end of the rack may be assembled through the inner seal of the power steering cylinder after the piston has been attached to the shank end of the rack bar (a practice well known in the art of manufacture of power steering gears). Also a conventional arcuate shaped rack support pad may be used and the entire rack bar may be finish ground by a through-feed centreless grinding process rather than by plunge grinding only the shank end.
When forging a D section rack, the cross-sectional area of toothed end of the rack bar is less than that of the shank end. It follows that either the rack bar blank must be reduced in diameter over that region later to be forged to form the toothed end or, alternatively, the excess metal must be extruded into side chambers adjoining the main die cavity, forming protrusions which can be removed by subsequent machining. Either approach will enable a power rack bar to pass through the seal and to be processed by through-feed centreless grinding. The present invention addresses the die construction for forging a D section rack employing such chambers. The general configuration of the forging die to be described may remain substantially that of the die described in U.S. Pat. Nos. 4,571,982 or 5,862,701 particularly in regard to the provision to grip the rack bar blank and to provide the necessary end constraints. However, the forming elements disclosed in the prior art are replaced by a pair of opposing die blocks each having a cavity in section corresponding to one or other half of the D section rack but including the above side chambers. The forging die disclosed in U.S. Pat. No. 4,571,982 also provides robust keys that engage as the upper and lower die blocks approach each other so that the die blocks are maintained in precise alignment.
Now, in the manufacture of Y section racks by warm forging, the very high pressures achieved ensure that the tooth cavities of the die are precisely filled over substantially all their length to within a fraction of a millimeter of the tops of the rack teeth. This is particularly important in the on-centre region of variable ratio racks used in power steering gears where the pressure angle is low and tooth contact with the pinion typically occurs only over the top one to two millimeters of the rack teeth. Poor die fill results in a reduction of the total length of the meshing pinion-rack contact lines and hence a corresponding reduction in effective contact ratio. In the case of a D section rack to which the present invention relates, this length of contact is already reduced as compared to that for Y section racks due to the aforementioned narrower rack teeth. The simple two forming element arrangement to be described must be capable of achieving the same very high pressures as the Y forming die.
Satisfactory die fill may be expressed as the meniscus or radius of the formed metal within the cross-section of the toothed cavity where, for example, the flank of the tooth cavity meets the bottom of the cavity. This radius is determined by the hydrostatic pressure of the formed metal within the die cavity. By experience it has been found that this should be of the order of 1100 MPa.
For this reason, the two element die construction illustrated in UK Patent 2088256, with its provision for "flash gutters" each side of the upper die chamber, is impractical in that metal would escape into these gutters before the desired hydrostatic pressure had been reached, particularly at the "hot forging" temperatures specified by the patent. According to this prior art patent specification, the initial hot forging process is followed at a later stage of manufacturing by a cold coining process when the final accuracy of form and tooth fill is achieved. Such a two stage process of forming the fine rack teeth may cause folds or other defects in the material unless precise compensation is made for the variation of temperature between hot forging and cold coining stages.
Likewise, the two die element construction illustrated in UK Patent 2108026, with its provision of a relief cavity whose volume is greater than that which would be filled by excess material displaced into it, would also be impractical in that metal would escape into the relief cavity before the desired hydrostatic pressure is reached to fully form the rack teeth.
Flash gutters are commonly provided in conventional forging dies to allow for the fact that the bar stock material used as blanks in forging operations is subject to a relatively wide variation of diameter, whereas in the processes described in U.S. Pat. No. 4,571,982 and 5,862,701, the blank is precision ground on its diameter to a fine tolerance. The chambers each side of the main die cavity according to the present invention have only a superficial resemblance to conventional flash gutters, and are primarily provided to prevent "pinching" of the metal which inevitably occurs at the joint line between mutually approaching die recesses having substantially semi-circular cross-sections. Other functions of these chambers are referred to later in the specification. Thus these chambers will be present whether or not the diameter of the blank has been reduced in diameter at the (yet to be formed) toothed end or not.
For these chambers to be effective, they must provide positive metal entrapment during the last stages of closing of the die in order to produce the aforementioned high final hydrostatic pressure. In fact, according to the present invention, during the last instant of closing, it is possible for metal from these chambers to re-enter the main die cavity in order to ensure complete die fill.
In one aspect the present invention consists of a die for forming the toothed end of a steering rack bar from a cylindrical blank by forging, the die comprising first and second die blocks relatively moveable to converge on the blank, said die blocks incorporating opposed generally semi circular recesses to accommodate said blank, one recess incorporating the obverse form of the teeth, said recesses defining between them a main cavity as said die blocks converge to their final closed position, subsidiary recesses in one or both die blocks at the joint line there between defining chambers at said final closed position, each side of said main cavity communicating with one of said chambers along all or most of the full longitudinal extent of said main cavity, characterised in that said chambers in cross-section incorporate stop means located laterally remote from said main cavity restricting further flow of blank material away from the main cavity, the volume of the chambers at said final closed position being substantially equal to the difference of volume between the rack bar blank and the steering rack bar as finish forged over the toothed end thereof in the main cavity.
It is preferred that said stop means comprises longitudinally extending abutments on said first die block which overlap respective juxtaposed abutments on said second die block as said final closed position is approached.
Alternatively, or in addition, it is preferred that said chambers are generally tapered in depth in a direction away from said main cavity sufficiently to inhibit further outward flow of blank material as said final closed position is approached.
Preferably, at least a portion of said blank material within said chambers is urged back towards said main cavity as said final closed position is approached.
Preferably each said longitudinal extending abutment on said first die block has a small clearance zone with said respective overlapped juxtaposed abutment on said second die block.